1. Field of the Invention
The present invention relates to a technique in which communication terminals in a plurality of different LANs (Local Area Networks) communicate over a WAN (Wide Area Network), in particular, to a communication technique between relay servers for relaying the communication.
2. Description of the Related Art
The communication terminals in the different LANs can directly communicate with each other over a WAN using a call control protocol such as SIP (Session Initiation Protocol) and DDNS (Dynamic Domain Name System). This is a technique referred to as a so-called VPN (Virtual Private Network), where different LANs at remote locations can be used as if the LANs are in a directly connected network.
Through the use of such technique, the LANs of a central office and a branch office can be connected, and the terminals connected to the respective LAN can communicate via the Internet.
Conventionally, a technique enabling the communication terminals connected to different LANs to communicate via the Internet has been known. In this technique, the different communication terminals connected to the different LANs log into a relay server connected to the Internet through the respective gateway to establish a communication path with the relay server. The communication terminals are thus able to communicate via the Internet using such a communication path. This technique is a technique that realizes communication between LANs using the relay server existing on the WAN.
When using the SIP, the account information of each communication terminal is generally registered in the SIP server. That is, the account information of the communication terminal is registered in the SIP server by issuing a REGISTER method to the SIP server from each communication terminal. When the communication terminal makes a communication request in which an account of another communication terminal is specified, the SIP server relays the communication for session establishment according to the registered account information.
When performing the communication using the SIP between a great number of LANs, a great number of account information of the communication terminals are registered in the SIP server, and thus the load of the server becomes very large. That is, the accounts of the communication terminals arranged in all the LANs were registered in the server that comprehensively manages the communication terminals in order to connect a great number of LANs and perform communication between the great number of LANs in the conventional art, and thus the load of the server is very large.
When the communication terminal existing in the network moves from one LAN to another LAN, the account information of the relevant communication terminal that is already registered becomes unnecessary, and thus the account information of the relevant communication terminal must be newly registered at the destination, in which case, management becomes difficult and troublesome, and furthermore, response to movement or the like of the communication terminal cannot be achieved in real time.